1. Field of the Invention
The present invention concerns a method with associated equipment for feeding two gases into and out of a multi-channel monolithic structure. The two gases will normally be two gases with different chemical and/or physical properties.
2. Description of Related Art
The gases, here called gas 1 and gas 2, are fed into channels for gas 1 and channels for gas 2 respectively. Gas 1 and gas 2 are distributed in the monolith in such a way that at least one of the channel walls is a shared or joint wall for gas 1 and gas 2. The walls that are joint walls for the two gases will then constitute a contact area between the two gases that is available for mass and/or heat exchange. This means that the gases must be fed into channels that are spread over the entire cross-sectional area of the monolith. The present invention makes it possible to utilize the entire contact area or all of the monolith's channel walls directly for heat and/or mass transfer between gas 1 and gas 2. This means that the channel for one gas will always have the other gas on the other side of its channel walls, i.e. all adjacent or neighbouring channels for gas 1 contain gas 2 and vice versa. The present invention is particularly applicable for making compact ceramic membrane structures and/or heat exchanger structures that must handle gases at high temperature. Typical applications are oxygen-conducting ceramic membranes, heat exchangers for gas turbines and heat exchanger reformers for production of synthetic gas.
A characteristic feature of multi-channel monolithic structures is that they consist of a body with a large number of internal longitudinal and parallel channels. The entire monolith with all its channels can be made in one operation, and the production technique used is normally extrusion. The monolith's channels are typically in the order of 1-6 mm in size, and the wall thickness is normally 0.1-1 mm. A multi-channel monolithic structure with channels of the sizes stated achieves a large surface area per volume unit. The typical values for monoliths with the channel sizes stated will be from 250 to 1000 m2/m3. Another advantage of monoliths is the straight channels, which produce low flow resistance for the gas. The monoliths are normally made of ceramic or metallic materials that tolerate high temperatures. This makes them robust and particularly applicable in high-temperature processes.
In industrial or commercial contexts, monoliths are mainly used where only one gas flows through all the channels in the monolith. The channel walls in the monolith may be coated with a catalyst that causes a chemical reaction in the gas flowing through. An example of this is monolithic structures in vehicle exhaust systems. The exhaust gas heats the walls in the monolith to a temperature that causes the catalyst to activate oxidation of undesired components in the exhaust gas.
Monolithic structures are also used to transfer heat from combustion gases or exhaust gases to incoming air for combustion processes. One method involves two gases, for example a hot and a cold gas, flowing alternately through the monolith. With such a method, for example, the exhaust gas can heat up the monolithic structure and subsequently emit heat to cold air. The air will then receive heat stored in the structure's material. When the heat is emitted from the material, the gas flow through the monolith changes back to exhaust gas, and the whole cycle is repeated. Such regenerative heat exchange processes with cycles in which there is alternation between two gases (one hot, one cold) in the same structure is not, however, suitable where mixture of the two gases is undesirable or where stable and continuous heat and/or mass exchange is desired. The industrial use of monoliths is limited mainly to applications in which only one gas flows through all the channels at the same time.
In the literature, a number of processes or applications are described in which monoliths can be used to advantage to transfer heat and/or mass between two different gas flows. Small-scale experimental tests have also been carried out with such processes. An example of this is production of synthetic gas (CO and H2). Synthetic gas is normally produced using steam reformation. This is an endothermic reaction in which methane and steam react to form synthetic gas. Such a process can be carried out to advantage in a monolith in which an exothermic reaction in adjacent channels supplies heat to the steam reformation.
Although it has been shown that it will be advantageous to use monoliths for mass and/or heat exchange between two gases in a number of applications, industrial use of monoliths for such applications is not very widespread. One of the most important points of complaint or reasons why monoliths are not used in this area is that the prior art technology for feeding the two gases into and out of the monolith's separate channels is complicated and not very suitable for scaling up (i.e. interconnection of several monolith units), particularly when the large number of channels in a monolith are taken into consideration.
German patent DE 196 53 989 describes a device and a method for feeding two gases into the monolith's channels through feed pipes. These feed pipes feed the two gases into the monolith's respective channels from the plenum chambers of the respective gases. The plenum chambers are outside each other, and the pipes from the outer chamber must be fed through the inner chamber and subsequently into the monolith's channels. Each pipe must be sealed in order to prevent leakage from the channels of the monolith and from lead-throughs in the walls of the plenum chambers.
When heated, the monolith, plenum walls, pipes and sealing material will expand, and, when cooled, they will contract. This increases the likelihood of crack formation and undesired leakage with mixture of the two gases as a consequence. This likelihood will increase with the number of pipe lead-throughs.
In DE 196 53 989, the inlet and outlet zone with the sealed pipes is cooled so that a low-temperature, flexible sealing material can be used and the risk of crack formation and leakage can be reduced. A cooling system will naturally make the monolithic structure more expensive and more complicated, particularly for applications on a large scale in which the monolith consists of many thousand channels and in which it is also necessary to use many monolithic structures in series and/or in parallel to achieve a sufficient surface area.
U.S. Pat. No. 4,271,110 describes another method for feeding two gases in and out. This method has the advantage that pipe in-feeds from the plenum chamber to the channels of the respective gases in the monolithic structure can be dispensed with completely. This is achieved by cutting parallel gaps down the ends of the monolith. These cuts or gaps lead into or out of the channels for one of the gases. The gaps cut then correspond to a plenum chamber for the row of channels that the gap cuts through. By sealing the gap's opening that faces out towards the end of the monolith, openings are created in the side wall of the monolith where one of the gases can enter or leave. The other gas will then enter or leave at the short end of the monolith in the remaining open channels. The biggest disadvantage of this method, apart from the necessary processing (cutting and sealing) of the monolithic structure itself, is that only half of the available area for mass and/or heat exchange can be utilized. For example, square channels for one gas and the other gas will have to lie in connected rows so that the channel structure for the two gases corresponds to a plate heat exchanger. If the channels for the two gases were distributed as in a check pattern, where the black fields correspond to channels for one gas and the white fields correspond to channels for the other gas, the maximum utilization of the area could be achieved because, in such a gas distribution pattern, all the walls of the channels for one gas would be joint or shared walls with those of the other gas. With gas channels for the same gas in a row as in U.S. Pat. No. 4,271,110, roughly only half of the channels' walls will be in contact with those of the other gas.